1. Field of the Invention
This invention relates to the construction of a plug valve and, more specifically, to a means for retaining the rotatable plug within the body of the valve and for preventing ingress of dirt or liquid into the interior of the valve.
2. Description of the Prior Art
There have heretofore been utilized a number of types of plastic plug valves for use in the gas distribution piping field. These valves are commonly buried underground and exposed to ground water and dirt which could interfere with valve operation. Some valve configurations such as those disclosed in U.S. Pat. Nos. 2,505,270, 2,604,293, 2,844,353, 3,103,948 and 3,133,723 are not suited for this environment because the base of the rotatable plug is exposed and could become jammed, preventing its rotation.
While some sealing at the bottom of the body is needed, those found in U.S. Pat. Nos. 2,493,966, 3,066,908 and 3,687,416 either complicate body or plug formation or are relatively expensive to provide. Further, although these valves are purportedly balanced by sealing rings around the plug above and below the flow passages, if the lower seal were to fail, having a pressure-tight area below the plug could present a problem. Fluid pressure could act on the base of the plug to force it from the body or cause it to bind interferring with rotation. This condition is explained in U.S. Pat. No. 3,350,057 which discloses a valve which is, by design, unbalanced. Retention of the plug within the body against the axial force created by the pressure buildup within a closed bottom of the body necessitates the use of a sturdy thrust washer and associated support structure.
It can also be seen from the various prior art balanced plug valves discussed above that there are a number of varied and complex configurations employed to provide a means for retaining the plug against axial movement within the body. Retention is obtained in these valves by enclosing the top and bottom of the body; entrapping a top enclosure between the plug and the handle; extending a set screw or pin into an aligned circumferential groove; using a pair of snap rings at the exposed top and bottom of the plug; or threading the plug into the bottom of the body.